A magneto-resistive (MR) element exhibits a change in electrical resistance as a function of external magnetic field. Such property allows MR elements to be used as magnetic field sensors, read heads in magnetic storage systems, and magnetic random-access-memories. In storage systems, the read head reads encoded information from a magnetic storage medium, which is usually a disc coated with a magnetic film. In a read mode, a magnetic bit on the disc modulates the resistance of the MR element as the bit passes below the read head. The change in resistance can be detected by passing a sense current through the MR element and measuring the voltage across the MR element. The resultant signal can be used to recover data from the magnetic storage medium. Depending on the structure of a device, the MR effect can fall into different categories, namely, anisotropic MR (AMR), giant MR (GMR), tunneling MR (TMR), colossal MR (CMR) and ballistic MR (BMR).
As the area densities of a hard disc increase beyond about 10 Gbit/inch2, AMR heads give way to GMR heads due to a GMR head's ability to produce a stronger signal. The GMR device currently manufactured in production by the data storage industry is the spin valve. The spin valve consists of a free layer the magnetization of which rotates with the external field, a non-magnetic metallic spacer layer (typically a Cu spacer or the like), a reference layer, a thin non-magnetic metallic layer (typically Ru), a pinned layer, and an antiferromagnetic pinning layer, such as PtMn or NiMn. The pinned layer has it its magnetization fixed along one direction by virtue of the exchange coupling between the magnetization of the pinned layer and the antiferromagnet, and the magnetization of the reference layer is in a direction anti-parallel to that of the pinned layer magnetization by virtue of the very strong antiferromagnetic coupling promoted between the pinned and reference layers by the thin Ru layer.
In a typical spin valve, the current flows in the plane of the metallic layers. This mode of operation is referred to as current-in-plane (CIP). The electrical resistance of a spin valve is a function of the angle between the magnetization of the free layer and of the reference layer. A sensor exhibits the largest resistance when the two layers are magnetized in anti-parallel directions, and the smallest when they are parallel. For proper operation and optimum sensitivity, the reference layer and free layer should be oriented at 90 degrees to one another. For standard spin valve designs this is accomplished by pinning the reference layer orientation out of the air bearing surface (ABS) of the sensor and biasing the free layer parallel to the ABS with permanent magnets positioned at the sides of the reader. The permanent magnets also act to stabilize the free layer response, ensuring a linear, hysteresis free response over the dynamic range of the free layer. In this configuration the signal output is determined by dynamic response of the free layer to magnetic bit transitions in the medium. The technology of GMR read heads has advanced so that it is possible to read from discs with information area densities up to 100 Gbit/inch2, beyond which point the sensitivity and output signal again becomes an issue.
One possible solution to the limitations of GMR heads is to use tunneling magnetoresistive (TMR) junctions. The standard TMR junction head design is very similar to a spin valve in the sense that it also consists of a free layer, a spacer layer, a reference layer, a Ru layer, a pinned layer, and an antiferromagnetic pinning layer. A major difference between TMR junctions and spin valves is that in the TMR junction the spacer layer is an oxide or semiconductor barrier as opposed to a conductor. Moreover, the electrical current in a TMR junction flows perpendicular to the plane of the films (CPP mode) as opposed to flowing in the plane for spin valve sensors (CIP). Since the spacer layer is an insulator or semiconductor in tunnel junctions, the electrons comprising the current tunnel through the barrier from the free layer to the reference layer. The magneto-resistance rises from the angular difference between the magnetization in the two magnetic layers in a way analogous to a spin valve. However, the TMR signal can be much larger than in spin valves, resulting in more amplitude and sensitivity of the device. Due to the unique nature of tunneling physics and the CPP current flow, TMR junctions offer more room for engineering design because the TMR signal and resistance are not directly related as they are in spin valves. In particular, the resistance of the junction depends only on the barrier thickness and junction area and not on the details of the rest of the stack. In theory, the magneto-resistance of a TMR junction depends only on the polarization of the free and reference layers and is independent of the junction area and the details of the other layers in the stack. In practice, for ultra-thin barriers there is a strong dependence on magnetoresistance as a function of barrier thickness due to processing defects in the barrier. The most common defects are known as pinholes and represent very small discontinuities in the tunneling barrier where the free and pinned layers are in direct contact. The current can shunt through these pinholes and reduce the resistance and magnetoresistance of the stack. In spite of defect limitations, the CPP TMR allows tremendous flexibility in head design, allowing independent optimization of the individual layers of the stack. Additionally, the geometry of the head can be optimized to achieve the best head performance without sacrificing head amplitude.
All types of MR elements typically include shields consisting of high permeability materials such as NiFe alloys. The function of the shields is to protect sensors from the stray magnetic fields originating from all magnetic bits on the medium, except the one just underneath the sensor. For spin valves where the current flows in CIP mode, the active sensor and leads are isolated from the shields by insulator material like metal oxide or nitride. This is to prevent current from leaking into the shields. As the linear density increases, the shield-to-shield spacing must be made smaller to adequately screen the flux from adjacent bits. Spin valves are not well suited to high linear density applications because they require half-gaps, which severely restricts how thin the shield-to-shield spacing can be. On the other hand, TMR junctions allow for the possibility of very small shield-to-shield spacing since the current flows in CPP mode so the half-gaps are not required. If the shields are used as electrodes to the tunnel junction, the shield-to-shield spacing consists of only the stack thickness. Therefore, tunnel junctions are superior to spin valves for high linear density applications. However, the antiferromagnetic pinning layer of a tunnel junction occupies a large proportion of the total stack thickness and provides a further limit to the shield-to-shield spacing for current spin valve and TMR designs. An alternate design to remove this layer would be highly desirable for further pushing the linear density limits of magnetic recording systems.
A further consideration when designing MR elements is the physical track density, or number of tracks per unit length in the radial direction of the disc, in memory media. As the track density increases, the width of the sensor layers has to decrease concomitantly in order for the sensor to fit well within one track. As the width of the sensor decreases, standard permanent magnet abutted junction designs have difficulty in stabilizing the free layer and maintaining an adequate output signal. There is generally a trade-off between providing a strong permanent magnet bias field to ensure a rotational, hysteresis-free response of the free layer and allowing the free layer to respond over a large enough portion of its range to ensure adequate output signal. The larger the permanent magnet bias field the more stable the free layer response, but at the cost of increased free layer stiffness and lower output signal. As the sensor width decreases, the window for providing adequate bias and sensitivity decreases rapidly. This is because a strong magnet field does not decay rapidly enough for narrow sensors, so the whole free layer is essentially pinned, resulting in very low amplitude. On the other hand, a weak permanent magnet bias field is not strong enough to stabilize the free layer edges resulting in head instabilities. Furthermore, shielding of stray magnetic fields originating from adjacent tracks also becomes an issue. The presence of permanent magnets on the sides of the stack prevents shielding of the free layer from adjacent tracks. For these reasons, the viability of current spin valve and TMR abutted junction head designs is very questionable as areal density targets exceed 100 Gb/in2.
Accordingly, there is a need for an MR element that is able to maintain high sensitivity with a relatively large output signal, while meeting the demands of increased linear bit density and track density. The present invention provides a solution to this and other limitations, and offers other advantages over the prior art.